5 - 13 = -8
But remember it is in the ||. This means the answer will, no matter what, be positive.
So the answer is |-8|, or +8.
A is the answer.
Now for an explanation of why the || makes it positive.
They are know as "absolute values." Absolute values will always, no matter what, be a positive number. These can be very tricky to deal with, but just remember to answer what is in the || first, and THEN do the absolute value. Remember that on a number line, the middle is always 0. And because -8 and 8 are both 8 spaces away from 0, the absolute value will be 8. This is why it is positive. You cannot be negative spaces away from something.